The present invention relates to the structure of a solenoid.
Most conventional type solenoids have a non-linear, force-current relationship. For example, at low current levels, the force changes caused by small current changes are smaller than the force changes caused by similar current changes at higher current levels. Such a force-current relationship is satisfactory when the solenoid is used as an on-off type actuator. However, where a proportional-type control function is needed, a linear-sloped force-current relationship would be desirable, such as where the force increases linearly as the current increases. In the past, various solenoid modifications have been utilized to provide particular force-displacement characteristics. For example, conical armatures and stops have been used to provide a uniform or constant force over a range of displacements, (see Mark's Standard Handbook for Mechanical Engineers, 7th edition, 1967, page 15-106, and U.S. Pat. Nos. 4,091,348 and 4,044,652). A similar uniform force-displacement relationship has been achieved in a solenoid made by Ledex, Inc. with a cylindrical steel shunt with a bevelled end. However, none of these arrangements provide a solenoid with the desired linear sloped force-current characteristic.